Le lundi 15 février 2021 à 13:10 +0100, Christian König a écrit :

Am 15.02.21 um 13:00 schrieb Thomas Zimmermann:

quoted

Hi

Am 15.02.21 um 10:49 schrieb Thomas Zimmermann:

quoted

Hi

Am 15.02.21 um 09:58 schrieb Christian König:

quoted

Hi guys,

we are currently working an Freesync and direct scan out from system 
memory on AMD APUs in A+A laptops.

On problem we stumbled over is that our display hardware needs to 
scan out from uncached system memory and we currently don't have a 
way to communicate that through DMA-buf.

Re-reading this paragrah, it sounds more as if you want to let the 
exporter know where to move the buffer. Is this another case of the 
missing-pin-flag problem?

No, your original interpretation was correct. Maybe my writing is a bit 
unspecific.

The real underlying issue is that our display hardware has a problem 
with latency when accessing system memory.

So the question is if that also applies to for example Intel hardware or 
other devices as well or if it is just something AMD specific?

I do believe that the answer is yes, Intel display have similar issue with
latency, hence requires un-cached memory.

Regards,
Christian.

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Best regards
Thomas

quoted

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For our specific use case at hand we are going to implement 
something driver specific, but the question is should we have 
something more generic for this?

For vmap operations, we return the address as struct dma_buf_map, 
which contains additional information about the memory buffer. In 
vram helpers, we have the interface drm_gem_vram_offset() that 
returns the offset of the GPU device memory.

Would it be feasible to combine both concepts into a dma-buf 
interface that returns the device-memory offset plus the additional 
caching flag?

There'd be a structure and a getter function returning the structure.

struct dma_buf_offset {
     bool cached;
     u64 address;
};

// return offset in *off
int dma_buf_offset(struct dma_buf *buf, struct dma_buf_off *off);

Whatever settings are returned by dma_buf_offset() are valid while 
the dma_buf is pinned.

Best regards
Thomas

quoted

After all the system memory access pattern is a PCIe extension and 
as such something generic.

Regards,
Christian.
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